Recovery of platinum group metals

Abstract

A process of recovering platinum group metals (PGMs) from a pregnant solution or leachate containing PGM values and base metals, typically a cyanide solution or leachate. A non-selective precipitation of the PGM values and base metals to form an insoluble precipitate comprising the PGM values and base metals is followed by selective leaching of the precipitate. The selective leaching forms a leach solution containing the base metals and a residue containing the PGM values, or a leach solution containing the base metals and PGM values and a depleted residue. The base metals are recovered from the leach solution and the PGM values are recovered from the residue or the leach solution, depending on the selective leaching. The non-selective precipitation of the PGM values and base metals is carried out by controlled reduction of the pH of the pregnant solution or leachate to a pH of about 2.

1. A process of recovering platinum group metals (PGMs) from a pregnant solution or leachate containing PGM values and base metals, the process including the steps of:
 a) non-selective precipitation of the PGM values and base metals to form an insoluble precipitate comprising the PGM values and base metals; 
 b) selective leaching of the precipitate to form (i) a leach solution containing the base metals and a residue containing the PGM values, or (ii) a leach solution containing the base metals and PGM values and a depleted residue; and 
 c) recovery of the base metals from the leach solution of b)(i) or b)(ii) and recovery of the PGM values from the residue of b)(i) or the leach solution of b)(ii), depending on the selective leaching in step b),
 wherein the pregnant solution or leachate containin the PGM values and base metals is a cyanide solution or leachate. 
 
 
     
     
       2. A process according to  claim 1 , wherein the non-selective precipitation of the PGM values and base metals in step a) is carried out by controlled reduction of the pH of the pregnant solution or leachate to within the range of about 0 to about 5. 
     
     
       3. A process according to  claim 2 , wherein the pH of the pregnant solution or leachate is reduced to within the range of about 1 to about 3. 
     
     
       4. A process according to  claim 3 , wherein the pH of the pregnant solution or leachate is reduced to about 2. 
     
     
       5. A process according to  claim 1 , wherein the selective leaching in step b) is a pressure leaching step in which the PGM values remain in the residue. 
     
     
       6. A process according to  claim 5 , wherein the PGM values are recovered from the residue by a fusion step or a further leach step to produce a concentrate rich in PGM values. 
     
     
       7. A process according to  claim 5 , wherein the pressure leaching step is conducted by first carrying out a caustic leach followed by an acid leach. 
     
     
       8. A process according to  claim 7 , wherein the PGM values are recovered from the residue by a reduction leach step to produce a high grade PGM concentrate. 
     
     
       9. A process according to  claim 1 , wherein the insoluble precipitate of step a) is fumed with acid, followed by selective leaching to form a leach solution containing both the PGM values as anions and the base metals as cations, and recovery of the PGM values on an ion exchange resin. 
     
     
       10. A process according to  claim 9 , wherein the resin is incinerated to form an ash rich in PGM values or eluted to recover the PGM values. 
     
     
       11. A process according to  claim 1 , wherein the insoluble precipitate of step a) is first calcined prior to selective leaching in step b), producing a PGM rich residue or concentrate. 
     
     
       12. A process according to  claim 1 , wherein the base metals are recovered from the leach solution of b)(i) or b)(ii) by precipitation. 
     
     
       13. A process according to  claim 1 , wherein any PGM values remaining in solution after the non-selective precipitation in step a) may be recovered by passing the solution through an ion exchange resin. 
     
     
       14. A process according to  claim 13 , wherein the resin is incinerated to form an ash rich in PGM values or eluted to recover the PGM values.